The invention relates, in general, to a method and apparatus for directional boring and, in particular to a bit system effective for directional boring in rock.
Directional boring machines for making holes through soil are well known. The directional borer generally includes a series of drill rods joined end to end to form a drill string. The drill string is pushed or pulled though the soil by means of a powerful hydraulic device such as a hydraulic cylinder. A spade, bit or chisel configured for boring having an angled steering face is disposed at the end of the drill string, and may include an ejection nozzle for water or drilling mud to assist in boring.
According to one known directional boring system, the drill bit is pushed through the soil without rotation in order to steer the tool by means of the angled face, which is typically a forwardly facing sloped surface. For rocky conditions, a row of teeth may be added to the drill bit and the bit operated in the manner described in Runquist et al. U.S. Pat. No. 5,778,991. Other toothed bits for directional boring through rock are shown in Cox U.S. Pat. No. 5,899,283, Skaggs U.S. Pat. No. 5,647,448 and Stephenson U.S. Pat. No. 5,799,740. As described in Runquist, in rock the drill can be steered cutting an arc or semicircular profile in the desired direction of travel. After the arc is bored, the tool is retracted and rotated back a like distance, or the rotation is completed with the head withdrawn so that no cutting occurs. The tool is then returned to engagement at the same location and the process is repeated. This process may be accomplished manually or by using an automated system such as the NAVTEC(copyright) drilling system used on the Vermeer NAVIGATOR(copyright) line of drilling machines.
Steering systems for use with these devices require keeping track of the angle of rotation of the sloped face of the bit and/or the teeth. According to one known system, a transmitter or sonde mounted in a tubular housing is mounted behind and adjacent to the bit and sends a signal that indicates the angle of rotation of the bit. The sonde is mounted in a predetermined alignment relative to the steering portion of the bit. See generally Mercer U.S. Pat. Nos. 5,155,442, 5,337,002, 5,444,382 and 5,633,589, Hesse et al. U.S. Pat. No. 5,795,991, and Stangl et al. U.S. Pat. No. 4,907,658. Mounting of the sonde in its housing has been accomplished by end loading or through a side opening which is closed by a door or cover during use, as illustrated in Lee et al. U.S. Pat. Nos. 5,148,880 and 5,253,721.
The xe2x80x9cduckbillxe2x80x9d style of bit, conventionally mounted directly on a forwardly sloped side face of the sonde housing, is inexpensive, generally easy to replace, and has the advantage of simplicity. Six bolts, which may be countersunk, hold the duckbill in place. The bit itself is little more than a flat steel plate the protrudes beyond the front end of the sonde housing. The bit may have teeth to aid in directional boring through rocky conditions. The bolts that hold the bit on, however, tend to loosen or fail under the large shear forces to which the bit is subjected, and once the bit breaks off, the bore must be discontinued and the drill head withdrawn.
A dual-purpose bit designed for directional boring through soil and horizontal drilling in rock, known as the Trihawk bit, is described in PCT Publication No. 00/11303, published Mar. 2, 2000. The Trihawk bit has three canted teeth set to cut a series of annular grooves which form the outer part of the borehole when drilling in rock. A mound or cone forms at the center of the borehole that is progressively broken down against the steering face as the bit advances. This bit is effective for drilling in dirt, soft rock and medium rock, but has limited drilling capability in hard rock. The present invention provides a bit which has greater durability and rock drilling power than the original Trihawk.
A bit for directional boring according to the invention includes a bit body having a frontwardly facing sloped face effective for steering the bit in dirt. The sloped face defines a steering plane that defines an acute included angle relative to a lengthwise axis of rotation of the bit. A connection is provided at the rear of the bit body permitting the bit to be removably mounted at the lead end of a drill string, and one or more internal passages are provided in the bit body for carrying a drilling fluid to a front end of the bit body. A first cutting tooth is mounted on the bit body and extends frontwardly from the bit body at a first angle that causes the first tooth to cut along a first circular path as the bit rotates. A second cutting tooth is mounted on the bit body and extends frontwardly from the bit body at a second angle that causes the second tooth to cut along a second circular path as the bit rotates, which second path has a diameter greater than the first circular path, and wherein a cutting tip at the front end of the second tooth is rearwardly offset from a cutting tip at the front end of the first tooth. In this manner, the second tooth effectively widen the smaller hole started by the first tooth, resulting in a highly effective rock drilling action. The two teeth may also be used to drill over a limited angle in order to steer the bit in rock, and the sloped face can be used in a known manner to push to steer when the bit is operating in dirt.
The invention further provides a method for directional drilling in rock with such a bit. The method includes the steps of bringing the bit into contact with a rock face so that a cutting tip of the first tooth engages the rock face, rotating the bit while applying pressure to the bit against the rock face so that the first tooth drills a hole in the rock face while a cutting tip of the second tooth remains free of contact with the rock face, and then continuing rotation of the bit while applying pressure to the bit against the rock face so that the second tooth drills into the rock face, widening the hole started by the first tooth. These and other aspects of the invention are described in detail below.